I started using Firefox full time about 6 months ago and for the life of me, I can't figure out why every time I click on a link to a Youtube video (usually here on CN), the Youtube screen comes up and the video portion stays black. It will never play. I always have to go to Chrome to watch it. I know its a setting, but I can't figure out which one.

I'm on the latest version, 19.0.2. I've got Youtube.com listed as an exception to "Block Pop-Ups".

I use WebRoot anti-virus and have all "http" websites as "low" security level, so I can't imagine that that's the problem.

Google Quickjava. It is an add on for Firefox. Shows up in the bottom toolbar as small boxes. You can click on each box independantly and enable/disable Javascript, Java, Silverlight, Cookies, Flash and a few more. Really neat add on.